As smart wireless terminals develop, standby time of a wireless terminal receives growing concern. A manner of increasing the standby time is using a battery of a large capacity in the wireless terminal. Another manner is maximizing work efficiency of a power-consuming device in the wireless terminal, for example, a power amplifier (PA). To ensure performance such as an adjacent channel leakage ratio (ACLR), the PA may work in a linear region. FIG. 1 is a schematic diagram of a work area of a PA, where a horizontal axis Vin represents an input signal of the PA, and a vertical axis Vout represents an output signal of the PA. It can be seen that the work area of the PA includes a linear region and a non-linear region. In the linear region, a gain (an amplification multiple) of the PA is a constant. However, in the non-linear region, that is, in a compression region shown in FIG. 1, the gain of the PA is no longer a constant, which means that a change of the input signal of the PA is no longer constant, and generally, the gain decreases compared with that in the linear region. It can be seen from FIG. 1 that when an input of the PA is greater than a specific extent, an output of the PA basically remains unchanged, that is, an output voltage is equal to or approximately equal to a supply voltage. The supply voltage in this case is a limit of the output voltage.
Generally, an output-signal voltage of the PA fluctuates greatly. Therefore, the supply voltage of the PA has to be high enough to cover all output signals within a dynamic voltage range. However, when an input-signal voltage and the output-signal voltage of the PA are relatively low, a very high supply voltage is also used, which causes a great waste of PA energy and very low power supply efficiency. A PA linearization technology enables the PA to work in the compression region, and to acquire ACLR performance similar to ACLR performance that is acquired when the PA works in the linear region. A typical technology thereof is ET (Envelope tracking). Using the technology, fine management is performed on the supply voltage of the PA, and the supply voltage is adjusted in real time as the input-signal voltage of the PA fluctuates, so that the power supply efficiency of the PA is greatly improved. Specifically, referring to FIG. 2, Vcc represents a supply voltage when a PA does not use the ET technology, and Vcc (ET) represents a supply voltage when the PA uses the ET technology. It can be seen that, when the PA uses the ET technology, the supply voltage of the PA fluctuates with an input-signal voltage, and does not need to remain as a constant voltage all the time, so that the supply voltage Vcc (ET) is equal to or approximately equal to an output voltage of the PA, that is, the supply voltage Vcc (ET) tracks a change of an input voltage of the PA, thereby effectively improving the power supply efficiency.
The ET technology enables, by dynamically adjusting the supply voltage of the PA, the PA to have a consistent signal gain at each output-signal power point, that is, a linear gain is achieved. FIG. 3 is a schematic diagram showing a relationship between a supply voltage and a gain of a PA in an ideal state. A horizontal axis represents an input of the PA, and a vertical axis represents an output of the PA. Multiple solid curves corresponding to Vcc1 to Vcc5 reflect a process in which an output signal of the PA changes with an input signal when a corresponding constant supply voltage is used. For example, a solid curve corresponding to Vcc1 shows an input/output signal relationship of the PA at the constant supply voltage Vcc1. There is a non-linear region in the solid curve. A thick dashed line shows an input signal/output signal relationship of the PA in ET, that is, the gain of the PA. For an input signal Vin of a different amplitude, a corresponding Vcc is used as the supply voltage. There are multiple choices for the supply voltage, for example, a successive increase from Vcc1 to Vcc5. When the PA works in the ET mode, because the supply voltage is no longer constant, the input signal and the output signal that are of the PA may maintain a linear relationship indicated by the thick dashed line. For example, when the input signal is Vin1, a value of a corresponding output-signal voltage at a corresponding gain is Vcc2, and in this case, Vcc2 may be used as the supply voltage; when the input signal is Vin2, a value of a corresponding output-signal voltage at a corresponding gain is Vcc3, and in this case, Vcc3 may be used as the supply voltage. The ET mode is used, and the supply voltage of the PA is adjusted, so that the supply voltage of the PA is approximately equal to the output-signal voltage of the PA, ensuring that an unnecessary electricity waste is reduced when the PA works.
In practical application, the input signal of the PA may be very weak. However, Vcc cannot be infinitely small, and there is only one minimum voltage shown by Vcc1 in FIG. 3. Therefore, a constant gain of the PA may not be achieved, and FIG. 3 is only an ideal case in the ET mode. As shown in FIG. 4, FIG. 4 is a schematic diagram showing another relationship between a supply voltage and a gain of a PA in the ET mode. The gain of the PA, that is, an input signal/output signal relationship may be divided into three parts. For details, refer to a thick dashed line in FIG. 4. Gains in a first segment, a second segment, and a third segment that are of the thick dashed line are different. The first segment indicates that the gain of the PA is constant, that is, a constant gain, and a minimum supply voltage Vcc1 may be used for power supply. The second segment is a transitional region in which an internal gain changes, and the Vcc1 may also be used for power supply. In the third segment, the gain is constant, and a changing Vcc may be used for power supply, that is, another voltage greater than Vcc1 is used for power supply. Therefore, in the ET mode, a system may obtain an output-signal voltage according to an input signal and the constant gain that are of the PA, so that the supply voltage Vcc of the PA is adjusted based on the output-signal voltage, it is implemented that a change of the Vcc tracks a change of the input signal of the PA, and the Vcc is approximately equal to the output-signal voltage.
Although gains in all the segments of the dashed line in FIG. 4 are the same, gains of the PA corresponding to output signals in different parts of an entire work area are inconsistent. Therefore, a digital baseband needs to be used for compensation processing. A typical compensation method is to use a DPD (Digital Predistortion, digital predistortion) technology. In a digital domain, a deviation between an expected gain that is set and an actual gain of the PA is corrected. That is, the input signal is processed before the input signal is amplified by the PA, so that the input signal changes to compensate non-linearity of the gain of the PA. FIG. 5 is a schematic diagram showing a structure of a radio frequency system based on digital predistortion in the ET mode. The radio frequency system includes a digital predistorter 51, configured to: receive a digital signal, where the digital signal is a baseband signal, that is, a digital domain signal before a radio frequency is sent. The digital predistorter 51 processes the digital signal to obtain a predistortion signal. The predistortion signal is sent to an ET system 52. The ET system 52 includes two paths. One is a signal path, including a DAC (digital-to-analog converter) 521, a high-pass filter 522, a frequency mixer 523, and a PA 524. A structure of the path is the same as that of a conventional transmitter. The other path of the ET system 52 is an envelope path, including an envelope signal calculator 525, a voltage converter 526, and a voltage generator 527. The digital-to-analog converter 521 is configured to perform digital-to-analog conversion on the predistortion signal to generate an analog signal. The high-pass filter 522 is configured to filter out noise in the analog signal. The frequency mixer 523 is configured to modulate the analog signal to a radio-frequency carrier fc to obtain a radio frequency signal input Vin. The PA 524 is configured to perform power amplification on the radio frequency signal input Vin to generate a radio frequency signal output Vout. The envelope signal calculator 525 is configured to calculate an envelope signal of the predistortion signal. The voltage converter 526 implements Env-Vcc conversion, that is, converts the envelope signal obtained by the envelope signal calculator 525 into a digital voltage, where the digital voltage is a representation of a real supply voltage of an actual PA in the digital domain. The voltage generator 527 is a digital-to-analog conversion apparatus, configured to convert the digital voltage into the real supply voltage of the PA, that is, an analog supply voltage Vcc. The supply voltage Vcc that is generated in the ET system 52 and that is of the PA may change with a signal envelope. In addition, the radio frequency signal output Vout generated by the PA 524 passes through a front-end module, for example, a duplexer 53, and is transmitted to an antenna 54. The antenna 54 transmits a radio frequency signal Vout. Because the voltage converter 526 converts the envelope signal into a corresponding digital voltage and the voltage generator 527 eventually generates the Vcc, a one-to-one correspondence between the Vcc and the envelope signal is implemented. The one-to-one correspondence may be stored in a form of a table. For details, refer to FIG. 6. FIG. 6 shows a lookup table (Lookup table, LUT) for Env-Vcc conversion, and reflects how the voltage converter 526 converts, based on the lookup table, an envelope signal into a corresponding supply voltage value.
FIG. 7 is a schematic diagram showing relationships between respective input/output signal amplitudes of a digital predistorter 51, an ET system 52, and an entire radio frequency system. The ET system 52 introduces predistortion of a signal gain of a PA, so that an input and an output of the PA are not completely linear, which means that the digital predistorter 51 performs inverse transformation on signal distortion of the ET system in advance, so that a predistortion gain between a DPD input and a DPD output, and a gain of the PA complement each other, and a system eventually implements a linear gain from a digital signal input into the digital predistorter 51 to an output Vout of the PA. Specifically, if a predistortion gain introduced by the digital predistorter 51 is A1 and an amplification gain of the PA is A2, for any one input/output of the radio frequency system, a gain is A0=A1×A2, where A0 is a constant value, indicating that the gain of the entire radio frequency system relative to a change of an input signal is constant.
A typical digital predistorter 51 is a table searcher, and the table searcher performs digital predistortion processing by using a lookup table algorithm. That is, signals of the DPD input and the DPD output are divided into a plurality of intervals (for example, 256 intervals), and a lookup table is formed based on these intervals, to implement a function for searching for the DPD output based on the DPD input. The lookup table is similar to the lookup table shown in FIG. 6 in form, and is different only in specific content of an entry. However, an implementation manner of the lookup table algorithm consumes an area of a chip and increases costs, and a real-time update of the lookup table is inconvenient. Another typical DPD implementation solution is to use a fitting polynomial manner for processing. That is, one polynomial is used for fitting an entire curve of the gain of the PA, and a gain of the PA corresponding to a specific input of the PA is obtained by using the curve, so that a DPD gain complementary to the gain of the PA is obtained, and eventually, it is implemented that a fitting is performed on a digital predistortion processing algorithm by using one polynomial. However, in the ET mode, the digital predistorter 51 needs to use one polynomial to fit an entire digital predistortion process, resulting in a complicated structure of the polynomial and excessive coefficients used in the polynomial. Therefore, a structure of the digital predistorter 51 is relatively complicated, and a fitting effect may not be ideally consistent with an actual gain curve.